Improved stats inspection

Previously, we printed column statistics for each column per row group via `parquet.metadata(uri)`.
With the new udf `parquet.column_stats(uri)`, we print stats for each column aggregated by row groups.

Stats for some of the types were printed in a text format that cannot be converted to
actual Postgres type. This PR also makes sure the output format is convertible to the actual
Postgres type.

Author: aykut-bozkurt

README.md

@@ -21,6 +21,7 @@ COPY table FROM 's3://mybucket/data.parquet' WITH (format 'parquet');
   - [Copy FROM/TO Parquet files TO/FROM Postgres tables](#copy-tofrom-parquet-files-fromto-postgres-tables)
   - [Inspect Parquet schema](#inspect-parquet-schema)
   - [Inspect Parquet metadata](#inspect-parquet-metadata)
+  - [Inspect Parquet column statistics](#inspect-parquet-column-statistics)
 - [Object Store Support](#object-store-support)
 - [Copy Options](#copy-options)
 - [Configuration](#configuration)
@@ -155,6 +156,29 @@ SELECT uri, encode(key, 'escape') as key, encode(value, 'escape') as value FROM
 (1 row)
 ```
 
+### Inspect Parquet column statistics
+You can call `SELECT * FROM parquet.column_stats(<uri>)` to discover the column statistics of the Parquet file, such as min and max value for the column, at given uri.
+
+```sql
+SELECT * FROM parquet.column_stats('/tmp/product_example.parquet')
+ field_id |         stats_min          |         stats_max          | stats_null_count | stats_distinct_count 
+----------+----------------------------+----------------------------+------------------+----------------------
+       19 | 2022-05-01 16:00:00        | 2022-05-01 16:00:00        |                0 |                     
+       15 |                            |                            |                2 |                     
+        3 | product 1                  | product 1                  |                0 |                     
+        2 | 1                          | 1                          |                0 |                     
+        0 | 1                          | 1                          |                0 |                     
+        6 | 1                          | 2                          |                1 |                     
+        7 | item 1                     | item 2                     |                1 |                     
+       16 |                            |                            |                2 |                     
+       12 |                            |                            |                2 |                     
+       18 | 2025-01-29 02:28:35.193773 | 2025-01-29 02:28:35.193773 |                0 |                     
+       11 | 1                          | 1                          |                1 |                     
+        8 | 1                          | 2                          |                1 |                     
+       17 |                            |                            |                2 |                     
+(13 rows)
+```
+
 ## Object Store Support
 `pg_parquet` supports reading and writing Parquet files from/to `S3` and `Azure Blob Storage` object stores.
 

src/parquet_udfs.rs

@@ -1,2 +1,3 @@
 pub(crate) mod metadata;
 pub(crate) mod schema;
+pub(crate) mod stats;

src/parquet_udfs/metadata.rs

@@ -1,4 +1,3 @@
-use ::parquet::file::statistics::Statistics;
 use pgrx::{iter::TableIterator, name, pg_extern, pg_schema};
 
 use crate::arrow_parquet::uri_utils::{
@@ -7,6 +6,8 @@ use crate::arrow_parquet::uri_utils::{
 
 #[pg_schema]
 mod parquet {
+    use crate::parquet_udfs::stats::{stats_max_value_to_pg_str, stats_min_value_to_pg_str};
+
     use super::*;
 
     #[pg_extern]
@@ -65,10 +66,12 @@ mod parquet {
                 let mut stats_null_count = None;
                 let mut stats_distinct_count = None;
 
+                let column_descriptor = column.column_descr();
+
                 if let Some(statistics) = column.statistics() {
-                    stats_min = stats_min_value_to_str(statistics);
+                    stats_min = stats_min_value_to_pg_str(statistics, column_descriptor);
 
-                    stats_max = stats_max_value_to_str(statistics);
+                    stats_max = stats_max_value_to_pg_str(statistics, column_descriptor);
 
                     stats_null_count = statistics.null_count_opt().map(|v| v as i64);
 
@@ -202,45 +205,3 @@ mod parquet {
         TableIterator::new(rows)
     }
 }
-
-fn stats_min_value_to_str(statistics: &Statistics) -> Option<String> {
-    match &statistics {
-        Statistics::Boolean(val_stats) => val_stats.min_opt().map(|v| v.to_string()),
-        Statistics::Int32(val_stats) => val_stats.min_opt().map(|v| v.to_string()),
-        Statistics::Int64(val_stats) => val_stats.min_opt().map(|v| v.to_string()),
-        Statistics::Int96(val_stats) => val_stats.min_opt().map(|v| v.to_string()),
-        Statistics::Float(val_stats) => val_stats.min_opt().map(|v| v.to_string()),
-        Statistics::Double(val_stats) => val_stats.min_opt().map(|v| v.to_string()),
-        Statistics::ByteArray(val_stats) => val_stats.min_opt().map(|v| match v.as_utf8() {
-            Ok(v) => v.to_string(),
-            Err(_) => v.to_string(),
-        }),
-        Statistics::FixedLenByteArray(val_stats) => {
-            val_stats.min_opt().map(|v| match v.as_utf8() {
-                Ok(v) => v.to_string(),
-                Err(_) => v.to_string(),
-            })
-        }
-    }
-}
-
-fn stats_max_value_to_str(statistics: &Statistics) -> Option<String> {
-    match statistics {
-        Statistics::Boolean(statistics) => statistics.max_opt().map(|v| v.to_string()),
-        Statistics::Int32(statistics) => statistics.max_opt().map(|v| v.to_string()),
-        Statistics::Int64(statistics) => statistics.max_opt().map(|v| v.to_string()),
-        Statistics::Int96(statistics) => statistics.max_opt().map(|v| v.to_string()),
-        Statistics::Float(statistics) => statistics.max_opt().map(|v| v.to_string()),
-        Statistics::Double(statistics) => statistics.max_opt().map(|v| v.to_string()),
-        Statistics::ByteArray(statistics) => statistics.max_opt().map(|v| match v.as_utf8() {
-            Ok(v) => v.to_string(),
-            Err(_) => v.to_string(),
-        }),
-        Statistics::FixedLenByteArray(statistics) => {
-            statistics.max_opt().map(|v| match v.as_utf8() {
-                Ok(v) => v.to_string(),
-                Err(_) => v.to_string(),
-            })
-        }
-    }
-}